Chemical mechanical polishing or planarizing a surface of an object may be desirable for several reasons. For example, chemical mechanical polishing is often used in the formation of microelectronic devices on a surface of a semiconductor wafer to provide a substantially smooth, planar surface suitable for subsequent fabrication processes such as photoresist coating and pattern definition.
During microelectronic fabrication, several structures that protrude from the wafer surface such as metal lines or that are etched into the surface such as vias and trenches may be formed. To smooth the wafer surface, a film of material may be deposited onto the wafer surface. The topography of the deposited film generally follows the topography of the underlying surface. However, the surface of the film may be smoothed using chemical mechanical polishing that preferentially removes material from the peaks on the surface compared to material located elsewhere on the wafer surface.
Chemical mechanical polishing may also be used to form microelectronic features. For example, a conductive feature such as a metal line or a conductive plug may be formed on a surface of a wafer by forming trenches and vias on the wafer surface, depositing conductive material over the wafer surface and into the trenches and vias, and removing the conductive material on the surface of the wafer using chemical mechanical polishing, leaving the vias and trenches filled with the conductive material.
A typical chemical mechanical polishing apparatus suitable for planarizing the semiconductor surface generally includes a wafer carrier configured to support, guide, and apply pressure to the wafer during the polishing process; a slurry containing abrasive particles and chemicals to assist removal of material from the surface of the wafer; and a polishing pad configured to assist in the material removal.
The wafer surface is generally polished by moving the surface of the wafer to be polished relative to the polishing pad in the presence of the slurry. In particular, the wafer is placed in the carrier such that the surface to be polished is exposed and faces downward. The wafer is then placed in contact with the pad (which has usually already been exposed to the slurry) and the pad and the wafer are moved relative to each other while slurry is continuously supplied to the polishing pad. Although the pad and the wafer may move relative to each other in a variety of ways, typically, the carrier is configured to cause the wafer to rotate about an axis and to translate back and forth across the polishing pad. Additionally, the pad is typically attached to a platen that is configured to rotate about an axis. As the wafer rotates, the outside diameter of the wafer moves across the wafer surface at a higher velocity than the surface at the wafer's inside diameter. To compensate for the higher velocity at the outside diameter, the wafer is often caused to translate across the surface of the pad, past the outside diameter of the pad, such that for a portion of the polishing process, the outside diameter of the wafer is not exposed to the polishing pad. In other words, a portion of the wafer surface is translated off the pad surface such that the surface near the inside diameter is polished and the surface near the outside diameter of the wafer is not polished for a portion of the polishing cycle.
The amount of material removed during or left remaining after the polishing process is typically controlled by, among other things, running the polishing process for a predetermined amount of time. The amount of time may be adjusted from run to run based on material removal rates from one or more previous polishing runs, wherein the removal rates are calculated by measuring the film thickness prior to polishing the wafer and measuring the remaining film thickness after the completion of the polishing process. The film thickness is generally measured using a device such as an x-ray fluorescence machine that is separate from the polishing apparatus. Consequently, the film thickness is typically measured before the wafer is placed on the polishing apparatus and again once the wafer is removed from the apparatus.
Because polishing rates often vary from run to run and wafer to wafer within a run, and the pre-polish material thickness may vary from wafer to wafer, this method often yields wafers that have been polished for too long or for not enough time, which may result in low device yield or poor device performance. Accordingly, improved methods for controlling an amount of material remaining on the wafer surface are desired.
One method for improving control over the amount of material remaining on the surface after a chemical mechanical polishing process includes measuring the film thickness as the wafer is be being polished and stopping the polishing process when a desired film thickness is reached. A method and an apparatus for measuring the film thickness during the polishing process is disclosed in U.S. Pat. No. Re. 34,425 issued to Schultz on Nov. 2, 1993. The method disclosed in the Schultz patent uses laser interferometry to measure a film thickness on a non-patterned die or dies located at the perimeter of the wafer surface. The measurements are taken each time a portion of the wafer oscillates off the polishing pad, allowing the portion to be exposed to the laser. This method is advantageous, because the polishing process may be stopped when a desired amount of material is removed from the wafer surface or when a desired film thickness is obtained rather than polishing the wafers for a predetermined amount of time. However, this method may be problematic in several regards. In particular, when lasers are used to measure film thicknesses, it is often difficult to differentiate the film thickness on the wafer surface and the thickness of the film in the trench. Accordingly, preferred methods in the Schultz patent include measuring the film thickness on a non-patterned portion of the wafer surface. Using non-patterned structures on the wafer surface reduces the number of devices that may otherwise be formed on the wafer. Accordingly, methods and apparatus for measuring a film thickness that do not require additional non-patterned areas on the wafer surface are desired.
In addition, because the measurements are taken while the wafer is being polished, the measurements are taken only at the very outside diameter of the wafer, typically across an arc portion of the wafer surface at the outside diameter. The surface area of the wafer where measurements are taken is limited, in part, because the wafers are forced against the polishing pad while the wafers are polished, and if the wafers extend too far beyond the outside diameter of the polishing platen, they are susceptible to breakage. Measuring film thickness only over a portion of the wafer at the outside diameter of the wafer may be undesirable because, among other reasons, the thickness at the portion of the outside diameter may not be indicative of the film thickness at an interior portion of the wafer. Accordingly, improved methods and apparatus for polishing workpieces that allow for measurement over a greater surface area of the workpiece without increasing workpiece breakage is desired.